Not Applicable.
This invention relates generally to exhaust systems, and more particularly to a stack exhaust system designed to minimize energy consumption.
The goal of stack exhaust systems is to exhaust toxic air from buildings to heights sufficient to avoid fresh air contamination and environmental pollution. Further considerations include prevention of condensed moisture in the stack and prevention of rain in the stack. Due to architectural and structural requirements, stack heights of stack exhaust systems are often required to be as short as possible. In order to satisfy environmental concerns and meet the architectural and structural requirements, stack exhaust systems are designed to exhaust air at sufficient velocities to create sufficient momentum to send the toxic air substantially higher than the stack and avoid contamination by the toxic air or recirculation of the toxic air.
FIG. 1 depicts a prior art stack exhaust system flow chart, which includes a stack 28, a fan 24, a make-up air damper 36, fume hoods 12, a static pressure sensor 20, and a controller 34. Because the fume hoods 12 are in standby mode most of the time, the total exhaust airflow from the fume hoods 12 is usually below the airflow for which the system is optimally designed.
The stack exhaust system of the prior art maintains the static pressure of the system by adjustment of the make-up air damper 36. That is, in order to maintain a constant discharge velocity, the system most operate at a constant volume. Therefore, when the total exhaust airflow from the fume hoods 12 is less than the design airflow, the controller opens the make-up air damper 36 to maintain the static pressure of the system.
The static pressure is typically measured at a common exhaust header 16 different than the mixing-joint 85 of the make-up air and the exhaust air. For example, the static pressure sensor 20 may be located either in the exhaust system farthest from the main riser or in the main plenum, because adequate static pressure must be maintained in the ductwork farthest from the exhaust fan plenum. As a result, the static pressure at the inlet of the fan is much greater under partial-exhaust airflow with make-up air conditions, than under full-exhaust airflow without make-up air conditions, and, thus, the airflow through the fan is higher than the design airflow. Consequently, fan energy consumption is higher under conditions of partial-exhaust airflow than under design conditions of full-exhaust airflow. In addition, fan motor overload is common due to the higher-than-design airflow.
The present invention is directed to a multi-stack exhaust system and method. It is desirable for the system to include at least one fume hood adapted for intake of toxic exhaust into an inlet duct in fluid communication with the at least one fume hood. In addition, it is desirable for the system to include at least two exhaust stacks, each stack having a return duct with a return damper and a discharge duct with a discharge damper. The return duct returns air and/or toxic exhaust to the inlet duct and the discharge duct discharges toxic exhaust to the environment. In another desirable embodiment the system further includes a fan, adapted to convey the toxic exhaust from the inlet duct to the at least two exhaust stacks. The system also may include a flow sensor for measuring the flow of toxic exhaust in the inlet duct, wherein the discharge dampers are adjusted such that the total flow from the discharge ducts is approximately equal to the flow of toxic exhaust in the inlet duct. It is also desirable that a first static pressure sensor for measuring the static pressure at the inlet duct, wherein an inlet duct static pressure set point is maintained by adjustment of the return dampers. Finally, it is desirable that a second static pressure sensor for measuring the static pressure at the at least two exhaust stacks, wherein an outlet duct static pressure set point is maintained by adjustment of the speed of the exhaust fan.
A desirable method for exhausting toxic exhaust includes generating toxic exhaust in at least one fume hood and passing the exhaust from the at least one fume hood to an inlet duct in fluid communication with the at least one fume hood. It is also desirable that the method include conveying the toxic exhaust to at least two exhaust stacks by a fan, each stack having a return duct with a return damper and a discharge duct with a discharge damper, wherein the return duct returns air and/or toxic exhaust to the inlet duct and the discharge duct discharges toxic exhaust to the environment. The method may also include measuring the flow of toxic exhaust in the inlet duct, wherein the discharge dampers are adjusted such that the total flow from the discharge ducts is approximately equal to the flow of toxic exhaust in the inlet duct. Also, it is desirable that the method include measuring the static pressure at the inlet duct, wherein an inlet duct static pressure set point is maintained by adjustment of the return dampers and measuring the static pressure at the at least two exhaust stacks, wherein an outlet duct static pressure set point is maintained by adjustment of the speed of the exhaust fan.
Additional objects, advantages, and novel features of the invention will be set forth in the description that follows and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.